<PAGE>
                                                                     Exhibit 4.8

      THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED
      UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR ANY OTHER SECURITIES
      LAWS. THESE SECURITIES HAVE BEEN ACQUIRED FOR INVESTMENT AND NOT WITH A
      VIEW TO DISTRIBUTION OR RESALE. SUCH SECURITIES MAY NOT BE OFFERED FOR
      SALE, SOLD, DELIVERED AFTER SALE, TRANSFERRED, PLEDGED OR HYPOTHECATED
      (EACH A "TRANSFER") IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT
      COVERING SUCH SECURITIES UNDER THE SECURITIES ACT OF 1933 AND ANY OTHER
      APPLICABLE SECURITIES LAWS, UNLESS THE HOLDER SHALL HAVE OBTAINED AN
      OPINION OF COUNSEL REASONABLY SATISFACTORY TO THE COMPANY THAT SUCH
      REGISTRATION IS NOT REQUIRED.

      THE TRANSFER AND VOTING OF THE SECURITIES OBTAINABLE UPON EXERCISE OF THIS
      STOCK PURCHASE WARRANT ARE RESTRICTED BY THE TERMS OF THE AMENDED AND
      RESTATED MANAGEMENT AND VOTING AGREEMENT, DATED AUGUST 17, 2001, AMONG THE
      COMPANY AND THE STOCKHOLDERS NAMED THEREIN, AND THE THIRD AMENDED AND
      RESTATED INVESTOR RIGHTS AGREEMENT, DATED AUGUST 17, 2001, AMONG THE
      COMPANY AND THE STOCKHOLDERS NAMED THEREIN AS SUCH AGREEMENTS MAY BE
      AMENDED FROM TIME TO TIME (THE "STOCKHOLDER AGREEMENTS"), COPIES OF WHICH
      MAY BE INSPECTED AT THE COMPANY'S PRINCIPAL OFFICE. THE COMPANY WILL NOT
      REGISTER THE TRANSFER OF SUCH SECURITIES ON THE BOOKS OF THE COMPANY
      UNLESS AND UNTIL THE TRANSFER HAS BEEN MADE IN COMPLIANCE WITH THE TERMS
      OF THE STOCKHOLDER AGREEMENTS.

No. SPW-1                                                       December 6, 2002

                             PTC THERAPEUTICS, INC.

                             STOCK Purchase Warrant

                                -----------------

      THIS STOCK PURCHASE WARRANT (this "WARRANT") certifies that, for value
received, Tularik Inc., or its registered assigns (the "HOLDER"), is entitled to
subscribe for and purchase from PTC Therapeutics, Inc., a Delaware corporation
(the "COMPANY"), up to 500,000 (subject to adjustment pursuant to Section 4
hereof) shares (the "SHARES") of the Company's Common Stock, par value $0.001
per share ("COMMON STOCK") at a price per share of $3.25 (subject to adjustment
pursuant to Section 4 hereof). This Warrant is being issued pursuant to

<PAGE>

PTC Warrant No. SPW-1                                               Confidential

that certain Securities Acquisition Agreement, dated as of the date hereof, by
and between the Company and Tularik Inc. (the "AGREEMENT").

      This Warrant is subject to the following terms and conditions:

      1. Vesting and Expiration of Shares. The exercise of this Warrant shall be
subject to the Company's achievement of the following milestones:

            (a) This Warrant shall become exercisable for up to 250,000 Shares
upon the Company's submission to the United States Food and Drug Administration
(the "FDA") in the U.S. or to an equivalent regulatory authority in a foreign
country of an Investigational New Drug Application (or the corresponding filing
or documentation of any foreign country) with respect to a Product Candidate
(the "FIRST IND FILING"). As used herein "PRODUCT CANDIDATE" shall mean any
product candidate for use in the Field. As used herein the "FIELD" shall mean
the therapeutic treatment of disease in humans or other animals by means of the
suppression of nonsense mutations. The right to purchase the above referenced
250,000 Shares shall expire and be of no force or effect if not exercised prior
to the tenth (10th) anniversary of receipt by Holder of written notice from the
Company of the First IND Filing.

            (b) This Warrant shall become exercisable for up to an additional
150,000 Shares upon the enrollment of the first human patient in a Phase II
Clinical Trial for any Product Candidate. As used herein "PHASE II CLINICAL
TRIAL" shall mean a clinical study conducted to evaluate the effectiveness of
the Product Candidate for a particular indication or indications in patients
with the disease or condition under study. The right to purchase the above
referenced 150,000 Shares shall expire and be of no force or effect if not
exercised prior to the eighth (8th) anniversary of receipt by Holder of written
notice from the Company of the commencement of the first Phase II Clinical
Trial.

            (c) This Warrant shall become exercisable for up to an additional
100,000 Shares upon any of the Company's Product Candidates receiving Marketing
Approval from the FDA or from an equivalent regulatory authority in a foreign
country. As used herein, "MARKETING APPROVAL" shall mean any approval necessary
for the marketing, promotion, sale or use of a Product Candidate as a
pharmaceutical product in a country. The right to purchase the above referenced
100,000 Shares shall expire and be of no force or effect if not exercised prior
to the fifth (5th) anniversary of receipt by Holder of written notice from the
Company of the first Marketing Approval of a Product Candidate.

            (d) Notwithstanding the foregoing, both Holder's right to continued
vesting of purchase rights as described in this Section and Holder's right to
purchase Shares pursuant to this Warrant shall expire and be of no force or
effect as of the 20th anniversary of the date of this Warrant.

      2. Method of Exercise; Payment.

            (a) Cash Exercise. Subject to the provisions of Section 1, the
purchase rights represented by this Warrant may be exercised by the Holder, in
whole or in part, at any time, or from time to time, by the surrender of this
Warrant (together with a duly executed notice of exercise (the "NOTICE OF
EXERCISE") in the form attached hereto as Exhibit A) at the Company's

                                     - 2 -
<PAGE>

PTC Warrant No. SPW-1                                               Confidential

principal offices, and by payment to the Company of an amount equal to the
Exercise Price multiplied by the number of the Shares being purchased, which
amount shall be paid in cash, by check or by wire transfer of immediately
available funds to an account designated by the Company. The person in whose
name any certificate representing the Shares issuable upon any exercise of this
Warrant shall be deemed to have become the holder of record of, and shall be
treated for all purposes as the record holder of, the Shares represented thereby
(and such Shares shall be deemed to have been issued) immediately prior to the
close of business on the date or dates upon which such surrender and payment are
made. As used herein, the term "PERSON" means any individual or any corporation,
partnership, trust, limited liability company or other entity or organization of
any kind.

            (b) Net Issue Exercise. In lieu of exercising this Warrant pursuant
to Section 2(a) hereof, the Holder may elect to receive a number of Shares equal
to the value (as determined below) of this Warrant (or the portion thereof being
exercised) by surrender of this Warrant at the Company's principal offices
together with a duly executed Notice of Exercise in which the appropriate
alternative is initialed by the Holder. In such event, the Company shall issue
to the Holder the number of Shares computed using the following formula:

                    X = Y (A-B)
                        -------
                           A

      Where X       =      the number of Shares to be issued to the Holder.

            Y       =      the number of Shares subject to this Warrant or, if
                           only a portion of this Warrant is being exercised,
                           the portion of this Warrant being exercised (at the
                           time of such calculation).

            A       =      the Fair Market Value of one Share (at the date of
                           such calculation).

            B       =      the Exercise Price (as adjusted to the date of such
                           calculation).

            (c) Fair Market Value. For purposes of this Section 2, the Fair
Market Value of one Share shall equal:

                  (i) the average of the closing sale prices of the Common Stock
(or any other security for which this Warrant is then exercisable) quoted on the
Nasdaq Stock Market or in the Over-The-Counter Market Summary or the closing
price quoted on any national securities exchange on which such securities are
listed, whichever is applicable, as published in the Western Edition of The Wall
Street Journal for the ten trading days immediately prior to the date of
determination of Fair Market Value (or, if no sales take place on any such
trading day, the average of the closing bid and asked prices on such trading
day); or

                  (ii) if the Common Stock (or any other security for which this
Warrant is then exercisable) is not traded on the Nasdaq Stock Market or
Over-The-Counter or on a national securities exchange, the Fair Market Value of
a Share shall be established in good faith by the Company's Board of Directors.

                                     - 3 -
<PAGE>

PTC Warrant No. SPW-1                                               Confidential

            (d) Stock Certificates. In the event of any exercise of the rights
represented by this Warrant, as promptly as practicable on or after the date of
exercise, the Company at its expense shall issue and deliver to the person or
persons entitled to receive the same a certificate or certificates representing
the number of Shares issued upon such exercise. In the event that this Warrant
is exercised in part, as promptly as practicable on or after the date of
exercise, the Company at its sole expense will execute and deliver a new Warrant
in the form of this Warrant exercisable for the number of Shares for which this
Warrant may then be exercised.

            (e) Taxes. The issuance of the Shares upon the exercise of this
Warrant, and the delivery of certificates or other instruments representing such
Shares, shall be made without charge to the Holder for any tax or other charge
of whatever nature in respect of such issuance and the Company shall bear any
such taxes in respect of such issuance.

      3. Stock Fully Paid; Reservation of Shares. All of the Shares issuable
upon the exercise of the rights represented by this Warrant will, upon issuance
and receipt of the Exercise Price therefor, be fully paid and nonassessable, and
free from all preemptive rights, rights of first refusal or first offer, taxes,
liens and charges of whatever nature, except as set forth in the Stockholder
Agreements (as defined in the Agreement) with respect to the issuance thereof.
During the period within which the rights represented by this Warrant may be
exercised, the Company shall at all times have authorized and reserved for
issuance a sufficient number of shares of its Common Stock to provide for the
full exercise of the rights represented by this Warrant.

      4. Certain Adjustments.

            (a) Special Definitions. For the purposes of this Section 4, the
following definitions shall apply:

                  (i) "ADDITIONAL SHARES OF COMMON STOCK" means all shares of
Common Stock issued (or, pursuant to Section 4(c), deemed to be issued) by the
Company after the Original Issue Date, other than:

                        (A) shares of Common Stock issued or issuable as a
dividend or distribution payable pro rata to all holders of Common Stock
(determined on a fully-diluted, as converted basis) of the Company;

                        (B) up to 3,000,000 shares of Common Stock or Options
issued or issuable to employees, consultants, officers, directors, advisors and
other persons performing services for the Company pursuant to the Company's 1998
Employee, Director and Consultant Stock Option Plan or other arrangement
approved by the Board of Directors of the Company; provided that, more than
3,000,000 shares of Common Stock or Options may be issued or issuable pursuant
to the Corporation's 1998 Employee, Director and Consultant Stock Option Plan
and not considered "Additional Shares of Common Stock" upon written consent of
the holders of a majority of the Preferred Stock;

                        (C) shares of Common Stock issued or issuable in
connection with the conversion or exercise of any Option or Convertible
Securities outstanding on the date hereof;

                                     - 4 -
<PAGE>

PTC Warrant No. SPW-1                                               Confidential

                        (D) shares of Common Stock issued or issuable under the
Agreement or in connection with the conversion or exercise of any Option or
Convertible Securities issued under the Agreement;

                        (E) up to 175,000 shares of Common Stock or Options
issued or issuable to institutional lenders in connection with the establishment
or maintenance by the Company of credit facilities, including equipment lease
facilities, approved in each case by a majority of the Company's Board of
Directors;

                        (F) shares of Common Stock issued or issuable pursuant
to a registered public offering, the closing of which is on or after the
Original Issue Date;

                        (G) up to 175,000 shares of Common Stock or Options
issued or issuable in connection with the sale of Common Stock or Convertible
Securities of the Company to any licensor of technology or patent rights to the
Company or to any collaborative partner or license with respect to the
development or commercialization of products;

                        (H) up to 1,000,000 shares of Common Stock or Options
issued or issuable in connection with the acquisition by the Company of another
corporation by merger, purchase of all or substantially all of its assets or
acquisition of all or substantially all of the capital stock of such
corporation; or

                        (I) shares of Common Stock that are exempted from the
definition of "Additional Shares of Common Stock" contained in the Shareholder
Documents (as defined below) by action of the holders of Preferred Stock as
provided for in the Shareholder Documents.

                  (ii) "CONVERTIBLE SECURITIES" means any evidences of
indebtedness, shares (other than the Common Stock), or other securities directly
or indirectly convertible into or exchangeable for the Common Stock; including
any series of Preferred Stock convertible by its terms into Common Stock.

                  (iii) "OPTION" means any right, option or warrant to subscribe
for, purchase or otherwise acquire Common Stock or Convertible Securities.

                  (iv) "ORIGINAL ISSUE DATE" means the date hereof.

                  (v) "PREFERRED STOCK" means any class of series of stock
designated as such under the Company's Restated Certificate of Incorporation, as
amended from time to time.

                  (vi) "SHAREHOLDER DOCUMENTS" shall mean collectively the
Company's Restated Certificate of Incorporation, as amended from time to time,
the Amended and Restated Management and Voting Agreement, dated as of August 17,
2001, by and among the Company, Tularik Inc. and certain other stockholders
named therein, and the Third Amended and Restated Investor Rights Agreement,
dated as of August 17, 2001, by and among the Company, Tularik Inc. and certain
other stockholders named therein.

                                     - 5 -
<PAGE>

PTC Warrant No. SPW-1                                               Confidential

            (b) No Adjustment of Exercise Price. No adjustment shall be made in
the Exercise Price as a result of the issuance of Additional Shares of Common
Stock or otherwise, unless the consideration per share determined pursuant to
Section 4(f) for an Additional Share of Common Stock issued or deemed to be
issued by the Company is less than the Exercise Price in effect on the date of,
and immediately prior to, the issue of such Additional Shares of Common Stock.

            (c) Issue of Options and Convertible Securities Deemed Issue of
Additional Shares of Common Stock. If the Company at any time or from time to
time after the Original Issue Date shall issue any Options or Convertible
Securities, or shall fix a record date for the determination of holders of any
class of securities entitled to receive any such Options or Convertible
Securities, then the maximum number of shares of the Common Stock (as set forth
in the instrument relating thereto without regard to any provision contained
therein for a subsequent adjustment of such number) issuable upon the exercise
of such Options or, in the case of Convertible Securities and Options therefor,
the conversion or exchange of such Convertible Securities, shall be deemed to be
Additional Shares of Common Stock issued as of the time of such issue or, in
case such a record date shall have been fixed, as of the close of business on
such record date, provided, that Additional Shares of Common Stock shall not be
deemed to have been issued unless the consideration per share determined
pursuant to Section 4(f) for such Additional Shares of Common Stock would be
less than the Exercise Price in effect on the date of and immediately prior to
such issue, or such record date, as the case may be, and provided, further, that
in any such case in which Additional Shares of Common Stock are deemed to be
issued:

                  (i) No further adjustment in the Exercise Price shall be made
upon the subsequent issue of Convertible Securities or shares of the Common
Stock upon the exercise of such Options or conversion or exchange of such
Convertible Securities;

                  (ii) If such Options or Convertible Securities by their terms
provide, with the passage of time or otherwise, for any increase or decrease,
other than any decrease due to anti-dilution provisions, in the consideration
payable to the Company, or decrease or increase, other than any increase due to
anti-dilution provisions, in the number of shares of the Common Stock issuable,
upon the exercise, conversion or exchange thereof, the Exercise Price computed
upon the original issue thereof (or upon the occurrence of a record date with
respect thereto), and any subsequent adjustments based thereon, shall, upon the
effectiveness of any such increase or decrease in consideration, or decrease or
increase in the number of shares, be recomputed to reflect such increase or
decrease in consideration, or decrease or increase in the number of shares,
insofar as it affects such Options or the right of conversion or exchange under
such Convertible Securities;

                  (iii) Upon the expiration of any such Options or any rights of
conversion or exchange under such Convertible Securities which shall not have
been exercised, the Exercise Price computed upon the original issue thereof (or
upon the occurrence of a record date with respect thereto) and any subsequent
adjustments based thereon shall, upon such expiration, be recomputed as if:

                                     - 6 -
<PAGE>

PTC Warrant No. SPW-1                                               Confidential

                        (A) In the case of Convertible Securities or Options for
Common Stock, the only Additional Shares of Common Stock issued were the shares
of the Common Stock, if any, actually issued upon the exercise of such Options
or the conversion or exchange of such Convertible Securities and the
consideration received therefor was the consideration actually received by the
Company for the issue of all such Options, whether or not exercised, plus the
consideration actually received by the Company upon such exercise, or for the
issue of all such Convertible Securities which were actually converted or
exchanged, plus the additional consideration, if any, actually received by the
Company upon such conversion or exchange; and

                        (B) In the case of Options for Convertible Securities,
only the Convertible Securities, if any, actually issued upon the exercise
thereof were issued at the time of issue of such Options, and the consideration
received by the Company for the Additional Shares of Common Stock deemed to have
been then issued was the consideration actually received by the Company for the
issue of all such Options, whether or not exercised, plus the consideration
deemed to have been received by the Company determined pursuant to Section 4(c)
upon the issue of the Convertible Securities with respect to which such Options
were actually exercised;

                  (iv) No recomputation pursuant to Section 4(c)(ii) or Section
4(c)(iii) above shall have the effect of increasing the Exercise Price to an
amount that exceeds the lower of (A) the Exercise Price prior to the original
adjustment for such deemed issuance, or (B) the Exercise Price that would have
resulted from any issuance of Additional Shares of Common Stock between the
original adjustment date and such recomputation date,

                  (v) In the case of any Options which expire by their terms not
more than thirty (30) days after the date of issue thereof, no adjustment of the
Exercise Price shall be made until the expiration or exercise of all such
Options, whereupon such adjustment shall be made in the same manner provided in
Section 4(c)(iii) above; and

                  (vi) If such record date shall have been fixed and such
Options or Convertible Securities are not issued on the date fixed therefor, the
adjustment previously made in the Exercise Price which became effective on such
record date shall be canceled as of the close of business on such record date,
and thereafter the Exercise Price shall be adjusted pursuant to this Section
4(c)(iii) as of the actual date of their issuance.

            (d) Stock Dividends, Stock Distributions and Subdivisions. In the
event the Company at any time or from time to time after the Original Issue Date
shall declare or pay any dividend or make any other distribution on the Common
Stock payable in Common Stock, or effect a subdivision of the outstanding shares
of Common Stock (by reclassification or otherwise than by payment of a dividend
in Common Stock), then and in any such event, Additional Shares of Common Stock
shall be deemed to have been issued:

                  (i) In the case of any such dividend or distribution,
immediately after the close of business on the record date for the determination
of holders of any class of securities entitled to receive such dividend or
distribution; or

                                     - 7 -
<PAGE>

PTC Warrant No. SPW-1                                               Confidential

                  (ii) In the case of any such subdivision, at the close of
business on the date immediately prior to the date upon which such corporate
action becomes effective.

                  If such record date shall have been fixed and such dividend
shall not have been fully paid on the date fixed therefor, the adjustment
previously made in the Exercise Price which became effective on such record date
shall be canceled as of the close of business on such record date, and
thereafter the Exercise Price shall be adjusted pursuant to this Section 4(d) as
of the time of actual payment of such dividend.

            (e) Adjustment of Exercise Price and Shares Upon Certain Events. If,
after the Original Issue Date, the Company shall issue Additional Shares of
Common Stock, including Additional Shares of Common Stock deemed to be issued
pursuant to Section 4(c) hereof, but excluding Additional Shares of Common Stock
deemed to be issued pursuant to Section 4(d) (which event is dealt with in
Section 4(g)) without consideration or for a consideration per share less than
the Exercise Price in effect immediately prior to such issue, then and in such
event, the Exercise Price shall be reduced, concurrently with such issue, to a
price (calculated to the nearest cent, provided that any adjustments not
required to be made by virtue of such rounding shall be carried forward and
taken into account in any subsequent adjustment) determined by multiplying the
Exercise Price in effect immediately prior to such issue by a fraction (x) the
numerator of which shall be (A) the number of shares of Common Stock outstanding
immediately prior to such issue (including shares of the Common Stock issuable
upon exercise or conversion of any outstanding Options or Convertible
Securities) plus (B) the number of shares of the Common Stock which the
aggregate consideration received by the Company for the total number of
Additional Shares of Common stock so issued would purchase at the Exercise Price
in effect immediately prior to such issue and (y) the denominator of which shall
be (A) the number of shares of the Common Stock outstanding immediately prior to
such issue (including shares of Common Stock issuable upon exercise or
conversion of any outstanding Options or Convertible Securities) plus (B) the
number of such Additional Shares of Common Stock so issued or deemed to be
issued. Upon each adjustment of the Exercise Price (other than pursuant to a
stock split, reverse stock split or similar transaction), the aggregate number
of Shares issuable upon exercise of the Warrant shall be adjusted to equal the
quotient obtained by dividing (a) the product resulting from multiplying (i) the
number of Shares issuable upon exercise of the Warrant and (ii) the Exercise
Price, in each case as in effect immediately before such adjustment, by (b) the
adjusted Exercise Price. The number of Shares vesting pursuant to Section 1
shall be proportionately adjusted in accordance with the adjustment made
pursuant to this Section 4(e). Upon the occurrence of each adjustment of the
Exercise Price, the Company at its expense shall promptly compute such
adjustment in accordance with the terms hereof and furnish to the Holder a
certificate setting forth such adjustment and showing in detail the facts upon
which such adjustment is based.

            (f) Determination of Consideration. For purposes of Section 4(e),
the consideration received by the Company for the issue of any Additional Shares
of Common Stock shall be computed as follows:

                  (i) Cash and Property: Such consideration shall:

                                     - 8 -
<PAGE>

PTC Warrant No. SPW-1                                               Confidential

                        (A) Insofar as it consists of cash, be computed at the
aggregate of cash received by the Company, excluding amounts paid or payable for
accrued interest or accrued dividends;

                        (B) Insofar as it consists of property other than cash,
be computed at the fair market value thereof at the time of such issue, as
determined in good faith by the Board of Directors; and

                        (C) In the event Additional Shares of Common Stock are
issued together with other shares or securities or other assets of the Company
for consideration which covers both, be the proportion of such consideration,
computed as provided in Sections 4(f)(i)(A) and 4(f)(i)(B) above, received in
respect of the Additional Shares of Common Stock, as determined in good faith by
the Board of Directors;

                  (ii) Options and Convertible Securities. The consideration per
share received by the Company for Additional Shares of Common Stock deemed to
have been issued pursuant to Section 4(c), relating to Options and Convertible
Securities, shall be determined by dividing:

                        (A) The total amount, if any, received or receivable by
the Company as consideration for the issue of such Options or Convertible
Securities, plus the minimum aggregate amount of additional consideration (as
set forth in the instruments relating thereto, without regard to any provision
contained therein for a subsequent adjustment of such consideration until such
subsequent adjustment occurs) payable to the Company upon the exercise of such
Options or the conversion or exchange of such Convertible Securities, or in the
case of Options for Convertible Securities, the exercise of such Options for
Convertible Securities and the conversion or exchange of such Convertible
Securities, by

                        (B) The maximum number of shares of the Common Stock (as
set forth in the instruments relating thereto, without regard to any provision
contained therein for a subsequent adjustment of such number until such
subsequent adjustment occurs) issuable upon the exercise of such Options or the
conversion or exchange of such Convertible Securities.

            (g) Adjustment for Stock Splits, Stock Dividends, Subdivisions,
Combinations or Consolidation of Common Stock. In the event that at any time or
from time to time after the Original Issue Date the outstanding shares of the
Common Stock shall be split, subdivided, combined or consolidated, by
reclassification or otherwise, into a greater or lesser number of shares of
Common Stock, and in the event that the Company shall issue shares of the Common
Stock by way of a stock dividend or other distribution to the holders of the
Common Stock, the Exercise Price and/or number of Shares covered by this Warrant
in effect immediately prior to such split, subdivision, stock dividend,
combination or consolidation shall, concurrently with the effectiveness of such
split, subdivision, stock dividend, combination or consolidation, be increased
or decreased proportionately.

            (h) Termination of Adjustment Rights. The provisions of this Section
4 (other than Section 4(g)) shall terminate immediately prior to the closing of
a firm commitment underwritten public offering pursuant to an effective
registration statement under the Securities

                                     - 9 -
<PAGE>

PTC Warrant No. SPW-1                                               Confidential

Act of 1933, as amended, covering the offer and sale of Common Stock for the
account of the Company to the public at an initial public offering price per
share of not less than $8.00 (subject to equitable adjustment in the event of
any stock dividend, stock split, combination, reorganization, recapitalization
or similar event involving a change in the Common Stock) with net proceeds to
the Company of not less than $40,000,000 (a "QUALIFIED INITIAL PUBLIC
OFFERING"). Thereafter, the provisions of this Section 4 shall be of no further
force or effect.

            (i) No Impairment. The Company shall not, by amendment of its
Restated Certificate of Incorporation or through any reorganization, transfer of
assets, consolidation, merger, dissolution, issue or sale of securities or any
other voluntary action, avoid or seek to avoid the observance or performance of
any of the terms to be observed or performed hereunder by the Company but shall
at all times in good faith assist in the carrying out of all the provisions of
hereof and in the taking of all such action as may be necessary or appropriate
in order to protect the rights of Holder against impairment.

      5. Notices.

            (a) In the event that the Company shall propose at any time:

                  (i) to declare any dividend or distribution upon its Common
Stock (or other stock or securities at the time receivable upon the exercise of
this Warrant) whether in cash, property, stock or other securities, whether or
not a regular cash dividend and whether or not out of earnings or earned
surplus;

                  (ii) to offer for subscription pro rata to the holders of any
class or series of its stock any additional shares of stock of any class or
series or other rights;

                  (iii) to effect any reclassification or recapitalization of
its Common Stock outstanding involving a change in the Common Stock;

                  (iv) to merge or consolidate with or into any other
corporation, or sell, lease or convey all or substantially all its property or
business, or to liquidate, dissolve or wind up; or

                  (v) to effect its Qualified Initial Public Offering;

then, in connection with each such event, the Company shall send to the Holder
(1) at least ten days' prior written notice of the date on which a record shall
be taken for such dividend, distribution or subscription rights (and specifying
the date on which the holders of Common Stock shall be entitled thereto) or for
determining rights to vote, if any; and (2) at least ten days' prior written
notice of the date when the same shall take place (and specifying the date, if
any is to be fixed, on which the holders of record of Common Stock shall be
entitled to exchange their Common Stock for securities or other property
deliverable upon the occurrence of such event). Notwithstanding the above, the
ten days' notice requirement may be shortened or waived upon the written consent
of the Holder.

                                     - 10 -
<PAGE>

PTC Warrant No. SPW-1                                               Confidential

            (b) Any written notice by the Company or the Holder required or
permitted hereunder shall be given in the manner provided in the Agreement;
provided, however the delivery of a Notice of Exercise shall be made in person,
by first class mail (postage pre-paid) or by nationally recognized overnight
courier and accompanied by this Warrant.

      6. Rights and Obligations.

            (a) The Shares shall be subject to the provisions of the Stockholder
Agreements in accordance with their respective terms, including, without
limitation, all applicable transfer restrictions set forth in the Stockholder
Agreements.

            (b) The Holder shall be entitled to incidental registration rights
with respect to the Shares on a pro rata basis with other security holders of
the Company solely to the extent such other security holders are entitled to
have their securities registered pursuant to Section 4.5 of the IR Agreement (as
defined in the Agreement); provided, however, that such Section 4.5 of the IR
Agreement shall be read without giving effect to the parenthetical phrase
"(other than pursuant to Sections 4.3 and 4.4)" solely with respect to the
Holder. Solely for purposes of determining the rights and obligations of the
Holder with respect to the incidental registration rights granted pursuant to
this Section 6(b), the Shares shall be treated as if they were "Registrable
Securities" (as defined in the IR Agreement) and the Holder shall be treated as
if it was a "Holder" (as defined in the IR Agreement) for purposes of Sections
4.6, 4.7, 4.8, 4.9, 4.10, 4.11, 4.12 and 4.13 of the IR Agreement; provided,
however, that except as expressly set forth herein, this Section 6(b) shall not
be interpreted in any manner that would expand the Holder's registration rights
beyond those incidental registration rights set forth in Section 4.5 of the IR
Agreement.

      7. Limitations of Transfer. Notwithstanding anything set forth herein to
the contrary, this Warrant may not be offered for sale, sold, transferred,
pledged, hypothecated or otherwise encumbered under any circumstances prior to
December 5, 2007; provided, however, the foregoing provision of this Section 7
shall not apply with respect to the transfer of this Warrant by the Holder to an
Affiliate (as defined in the Termination and License Agreement, dated the date
hereof, by and between the Company and Tularik Inc. (the "TERMINATION AND
LICENSE AGREEMENT")) or in connection with the sale or merger of all or
substantially all of the Holder's business, so long as such transfer is
otherwise permissible under all applicable securities laws, the terms of this
Warrant and any other obligations or limitations to which the Holder or this
Warrant is subject.

      8. Legend. Each certificate evidencing the Shares issued upon exercise of
this Warrant, or transfer of such shares (other than a transfer registered under
the Securities Act of 1933, as amended (the "SECURITIES ACT"), or any subsequent
transfer of shares so registered) shall be stamped or imprinted with a legend
substantially in the following form:

            THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN
            REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR ANY
            OTHER SECURITIES LAWS. THESE SECURITIES HAVE BEEN ACQUIRED FOR
            INVESTMENT AND NOT WITH A VIEW TO DISTRIBUTION OR RESALE. SUCH

                                     - 11 -
<PAGE>

PTC Warrant No. SPW-1                                               Confidential

            SECURITIES MAY NOT BE OFFERED FOR SALE, SOLD, DELIVERED AFTER SALE,
            TRANSFERRED, PLEDGED OR HYPOTHECATED (EACH A "TRANSFER") IN THE
            ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT COVERING SUCH
            SECURITIES UNDER THE SECURITIES ACT OF 1933 AND ANY OTHER APPLICABLE
            SECURITIES LAWS, UNLESS THE HOLDER SHALL HAVE OBTAINED AN OPINION OF
            COUNSEL REASONABLY SATISFACTORY TO THE COMPANY THAT SUCH
            REGISTRATION IS NOT REQUIRED.

            THE TRANSFER AND VOTING OF ANY OF THE SECURITIES REPRESENTED BY THIS
            CERTIFICATE ARE RESTRICTED BY THE TERMS OF THE AMENDED AND RESTATED
            MANAGEMENT AND VOTING AGREEMENT, DATED AUGUST 17, 2001, AMONG THE
            COMPANY AND THE STOCKHOLDERS NAMED THEREIN AND THE THIRD AMENDED AND
            RESTATED INVESTOR RIGHTS AGREEMENT, DATED AUGUST 17, 2001, AMONG THE
            COMPANY AND THE STOCKHOLDERS NAMED THEREIN, AS SUCH AGREEMENTS MAY
            BE AMENDED FROM TIME TO TIME (THE "STOCKHOLDER AGREEMENTS"), COPIES
            OF WHICH MAY BE INSPECTED AT THE COMPANY'S PRINCIPAL OFFICE. THE
            COMPANY WILL NOT REGISTER THE TRANSFER OF SUCH SECURITIES ON THE
            BOOKS OF THE COMPANY UNLESS AND UNTIL THE TRANSFER HAS BEEN MADE IN
            COMPLIANCE WITH THE TERMS OF THE STOCKHOLDER AGREEMENTS.

      9. Removal of Legend. Upon request of a holder of a certificate with the
legends required by Section 8 hereof, the Company shall issue to such holder a
new certificate therefor free of any transfer legend, if, with such request, the
Company shall have received an opinion of counsel satisfactory to the Company in
form and substance to the effect that any transfer by such holder of the shares
evidenced by such certificate will not violate the Securities Act, any
applicable state securities laws, or any applicable provision of any Stockholder
Agreement.

      10. Fractional Shares. No fractional shares will be issued in connection
with any exercise hereunder. All shares of Common Stock (including fractions
thereof) issuable upon exercise of this Warrant by the Holder shall be
aggregated for purposes of determining whether the exercise would result in the
issuance of any fractional share. Any fraction of a share resulting from any
calculation will be rounded down to the next whole share.

      11. Representations and Warranties of the Company. The Company hereby
represents and warrants to the Holder as follows:

            (a) This Warrant has been duly authorized and executed by the
Company and is a valid and binding obligation of the Company enforceable in
accordance with its terms;

            (b) Prior to the date that this Warrant becomes exercisable, the
Shares will have been duly authorized and reserved for issuance by the Company
and, when issued in

                                     - 12 -
<PAGE>

PTC Warrant No. SPW-1                                               Confidential

accordance with the terms hereof, will be validly issued, fully paid and
nonassessable, and free from all preemptive rights, rights of first refusal or
first offer, taxes, liens and charges of whatever nature, except as set forth in
the Stockholder Agreements; and

            (c) The execution and delivery of this Warrant are not, and the
issuance of the Shares upon exercise of this Warrant in accordance with the
terms hereof will not be, inconsistent with the Company's Restated Certificate
of Incorporation, as then in effect, and its bylaws, as then in effect.

      12. Representations and Warranties by the Holder. The Holder represents
and warrants to the Company as follows:

            (a) This Warrant is being acquired for its own account, for
investment and not with a view to, or for resale in connection with, any
distribution thereof within the meaning of the Securities Act. Upon exercise of
this Warrant, the Holder shall, if so requested by the Company, confirm in
writing, in a form reasonably satisfactory to the Company, that the Shares
issuable upon exercise of this Warrant are being acquired for investment and not
with a view toward distribution or resale that would violate the Securities Act.

            (b) The Holder understands that this Warrant and the Shares have not
been registered under the Securities Act by reason of their issuance in a
transaction exempt from the registration and prospectus delivery requirements of
the Securities Act pursuant to Section 4(2) thereof, and that they must be held
by the Holder indefinitely, and that the Holder must therefore bear the economic
risk of such investment indefinitely, unless a subsequent disposition thereof is
registered under the Securities Act or is exempted from such registration.

            (c) The Holder has such knowledge and experience in financial and
business matters that it is capable of evaluating the merits and risks of the
purchase of this Warrant and the Shares purchasable pursuant to the terms of
this Warrant and of protecting its interests in connection therewith.

            (d) The Holder is able to bear the economic risk of the purchase of
the Shares pursuant to the terms of this Warrant.

      13. Rights of Stockholders. Subject to Sections 4 and 5 hereof, no Holder,
as such, shall be entitled to vote or receive dividends or be deemed the holder
of the Shares or any other securities of the Company that may at any time be
issuable on the exercise hereof for any purpose, nor shall anything contained
herein be construed to confer upon the Holder, as such, any of the rights of a
stockholder of the Company or any right to vote for the election of directors or
upon any matter submitted to stockholders at any meeting thereof, or to give or
withhold consent to any corporate action (whether upon any recapitalization,
issuance of stock, reclassification of stock, change of par value,
consolidation, merger, conveyance, or otherwise) or to receive notice of
meetings, or to receive dividends or subscription rights or otherwise until this
Warrant shall have been exercised and the Shares purchasable upon the exercise
hereof shall have become issuable, as provided herein.

                                     - 13 -
<PAGE>

PTC Warrant No. SPW-1                                               Confidential

      14. Miscellaneous.

            (a) This Warrant shall be governed by and construed for all purposes
under and in accordance with the laws of the State of Delaware without regard to
principles of conflicts of law.

            (b) The headings in this Warrant are for purposes of reference only,
and shall not limit or otherwise affect any of the terms hereof.

            (c) The representations, warranties, covenants and conditions of the
respective parties contained herein or made pursuant to this Warrant shall
survive the execution and delivery of this Warrant.

            (d) The terms of this Warrant shall be binding upon and shall inure
to the benefit of any successors or assigns of the Company and of the Holder or
holders hereof and of the Shares issued or issuable upon the exercise hereof.

            (e) This Warrant, together with the Agreement and the Termination
and License Agreement, constitute the full and entire understanding and
agreement between the parties with regard to the acquisition of this Warrant and
the Shares (as defined in the Agreement).

            (f) The Company shall not, by amendment of its Restated Certificate
of Incorporation or bylaws, or through any other means, directly or indirectly,
avoid or seek to avoid the observance or performance of any of the terms of this
Warrant and shall at all times in good faith assist in the carrying out of all
such terms and in the taking of all such action as may be necessary or
appropriate in order to protect the rights of the Holder against impairment.

            (g) Upon receipt of evidence reasonably satisfactory to the Company
of the loss, theft, destruction or mutilation of this Warrant and, in the case
of any such loss, theft or destruction, upon delivery of an indemnity agreement
reasonably satisfactory in form and amount to the Company or, in the case of any
such mutilation, upon surrender and cancellation of such Warrant, the Company,
at its expense, will execute and deliver to the Holder, in lieu thereof, a new
Warrant of like date and tenor.

            (h) Except as otherwise provided herein, this Warrant and any
provision hereof may be amended, waived or terminated only by an instrument in
writing signed by the Company and the Holder.

                            [Signature Page Follows]

                                     - 14 -
<PAGE>

PTC Warrant No. SPW-1                                               Confidential

      IN WITNESS WHEREOF, the Company has caused this Warrant to be signed by
its duly authorized officer.

                                             PTC THERAPEUTICS, INC.

                                             By /s/ MARK E. BOULDING
                                                ________________________________
                                                Mark E. Boulding
                                                Senior Vice President, Business
                                                Development and Legal; Secretary
                                                of the Corporation

Acknowledged and Accepted:

TULARIK INC.

By /s/ J. Rieflin
  _________________________________
Name:  J. Rieflin
Title: Vice President

                                     - 15 -
<PAGE>

                                                                    Confidential

                                                                       Exhibit A

                               NOTICE OF EXERCISE

TO: PTC Therapeutics, Inc.

      Attention: Legal Department

      1. The undersigned hereby elects to purchase _______________ (leave blank
if you choose Alternative No. 2 below) shares of Common Stock, par value $0.001
per share, of PTC Therapeutics, Inc. pursuant to the terms of this Warrant, and
tenders herewith payment of the purchase price of such shares in full. (Initial
here if the undersigned elects this alternative). _________

      2. In lieu of exercising the attached Warrant for cash, the undersigned
hereby elects to effect the net issuance provision of Section 2(b) of this
Warrant and receive ____________ (leave blank if you choose Alternative No. 1
above) shares of Common Stock, par value $0.001 per share, of PTC Therapeutics,
Inc. pursuant to the terms of this Warrant. (Initial here if the undersigned
elects this alternative). ___________

      3. Please issue a certificate or certificates representing said securities
in the name of the undersigned or in such other name as is specified below:

                    _________________________________________
                                     (Name)

                    _________________________________________

                    _________________________________________
                                    (Address)

      4. The undersigned hereby represents and warrants that the aforesaid
securities are being acquired for the account of the undersigned for investment
and not with a view to, or for resale, in connection with the distribution
thereof, and that the undersigned has no present intention of distributing or
reselling such shares and all representations and warranties of the undersigned
set forth in Section 12 of the attached Warrant are true and correct as of the
date hereof.

                                       _________________________________________
                                       (Signature and Date)